netdev - Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

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Message-ID: <CACAyw9-Ni4UaZuUOJHpO2xm2y6Dwtcn98gWsYW1ShmQg-W8TxQ@mail.gmail.com>
Date:   Wed, 29 Sep 2021 13:32:15 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        tirthendu.sarkar@...el.com
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

On Wed, 29 Sept 2021 at 13:25, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> > Then use the already existing (right ;P) inlining to do the following:
> >
> >    if (md->ptr + args->off != ret_ptr)
> >      __pointer_flush(...)
>
> The inlining is orthogonal, though, right? The helper can do this check
> whether or not it's a proper CALL or not (although obviously for
> performance reasons we do want it to inline, at least eventually). In
> particular, I believe we can make progress on this patch series without
> working out the inlining :)

Yes, I was just worried that your answer would be "it's too expensive" ;)

> > This means that __pointer_flush has to deal with aliased memory
> > though, so it would always have to memmove. Probably OK for the "slow"
> > path?
>
> Erm, not sure what you mean here? Yeah, flushing is going to take longer
> if you ended up using the stack pointer instead of writing directly to
> the packet. That's kinda intrinsic? Or am I misunderstanding you?

I think I misunderstood your comment about memcpy to mean "want to
avoid aliased memory for perf reasons". Never mind!

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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